You proved my point. No one is guaranteed an upgrade loaner, much less a like-for-like.
You still cannot tell us of any time you got a like-for-like replacement, or where this is common practice. You got 'the loaner they had on hand', which was the same one (i.e. a C-class or equivalent) they would have given you if your original car was a S560.
Not only do I expect that sometimes Mercedes might give you a Chevy Malibu, I've experienced it first hand. When my local BMW dealership runs out of BMW loaners, they give you credit to use at their on-site Enterprise. What does Enterprise give you? Whatever the have left sitting on the lot, and they're never BMWs.
Apple gave you the loaner they had on hand, which is exactly what Mercedes did. In the Mercedes example, you came out ahead. With the Apple example, it was 'horrific'.